1. Field of the Invention
The present invention relates to a mold suitable for a stamper for a discrete track medium, a stamper for a patterned medium, a master disk for magnetic transfer, and the like; a method for producing the mold; and a magnetic recording medium.
2. Description of the Related Art
In recent years, for magnetic recording media, remarkable increase in surface recording density has been achieved by means of improvements such as reduction in the size of magnetic particles constituting recording layers thereof, alteration of materials and minuteness in head processing, and, for the future too, further increase in surface recording density is hoped for.
However, problems such as limitations of head processing, incorrect recording of information to another track situated next to a target recording track stemming from the widening of a magnetic field, and crosstalk at the time of reproduction have become conspicuous, and thus surface recording density can hardly be increased by conventional improved methods anymore. Accordingly, a discrete track medium and a patterned medium, in which a concavo-convex pattern is provided or a magnetic layer is divided to reduce magnetic interaction caused between tracks, have been proposed as potential magnetic recording media that make it possible to increase surface recording density further (refer to Japanese Patent Application Laid-Open (JP-A) No. 2000-195042). In the case of the discrete track medium, a recording layer is created in the form of a track pattern in a data region. In the case of the patterned medium, a recording layer is created in the form of a pattern such as bits. Also, in the case of the discrete track medium and the patterned medium, the recording layers are created in the form of servo patterns.
It is proposed that concavo-convex patterns in such magnetic recording media be formed in accordance with a nanoimprinting lithography method (an imprinting method whereby a concavo-convex pattern of a nanometer size is formed; hereinafter otherwise referred to as “imprinting method”), in which by pressing a mold (hereinafter otherwise referred to as “stamper”) having a concavo-convex pattern of a nanometer size against a resinous layer on a base material and so transferring the concavo-convex shape of the mold onto the resinous layer, the concavo-convex pattern of a nanometer size is formed on the base material (refer to Japanese Patent Application Laid-Open (JP-A) No. 2005-353164).
The stamper is typically produced by an electroforming step in which an original plate where information has been formed as a concavo-convex pattern is electroformed, then a metal plate formed of an electroforming layer is laid on the original plate and the concavo-convex pattern is transferred onto the metal plate surface, and a peeling step in which the metal plate is peeled off the original plate.
However, conventional stampers produced by the above-mentioned method are not necessarily flat but often warped or distorted. Hence, there is a problem that highly accurate concavo-convex patterns cannot be formed, as concavo-convex patterns formed are deformed or distorted.
Also, as to magnetic disks (hard disks) used for hard disk drives that have been rapidly spreading in recent years, format information and address information are typically written to them after they have been delivered from magnetic disk makers to drive makers and before they are incorporated in the drives. Although this writing can be conducted by means of a magnetic head, a method of transferring format information and address information at one time by means of a mold (hereinafter otherwise referred to as “master disk”) to which the format information and the address information have been written is more efficient and desirable.
As to this magnetic transfer method of transferring format information and address information at one time, when a master disk and a transfer target disk (slave disk) are closely attached to each other, by applying a transfer magnetic field to either or both surfaces thereof with a magnetic field generator such as an electromagnetic device or permanent magnet device, information (e.g. a servo signal) which the master disk has is magnetically transferred onto the slave disk. And it is very important that the master disk and the slave disk be closely attached to each other evenly with no space in between so as to conduct the magnetic transfer accurately.
Incidentally, for the master disk used in this magnetic transfer method, a master disk in which a concavo-convex pattern corresponding to an information signal is formed on the surface of a substrate, and the surface of this concavo-convex pattern is covered with a magnetic layer is normally used, as in Japanese Patent Application Laid-Open (JP-A) No. 2001-256644, for example. This master disk for magnetic transfer is typically produced by covering the surface of the concavo-convex pattern with the magnetic layer after undergoing an electroforming step in which an original plate where information has been formed as a concavo-convex pattern is electroformed, then a metal plate formed of an electroforming layer is laid on the original plate and the concavo-convex pattern is transferred onto the metal plate surface; a peeling step in which the metal plate is peeled off the original plate; and a punching step in which the metal plate peeled off is cut by punching into a predetermined size.
However, conventional master disks produced by the above-mentioned method are not necessarily flat but often warped or distorted as a result of being deformed at the times of processes exemplified by the peeling step in which metal plates are peeled off original plates and the punching step in which the metal plates are cut by punching into predetermined sizes. In addition, warpage and distortion may be caused by internal stress of electroforming layers in a photo-etching step as well. When a master disk is warped or distorted as just described, there is a problem that it is impossible to improve the state in which the master disk and a slave disk are closely attached to each other and thus impossible to perform highly accurate magnetic transfer.
In attempts to solve the forgoing problems, the back surface of a master disk is provided with a buffer (cushion material), adhesive pressure is increased, air on the interface where the master disk and a slave disk are closely attached is removed by means of vacuum suction, and so forth, in order for the master disk to be attached to the slave disk more closely. However, despite these attempts, the problems with adhesion have not completely been solved; in essence, it is necessary to improve the flatness of the master disk by removing the warpage or distortion of the master disk. Also, there is a possibility that increasing adhesive pressure may damage or deform a concavo-convex pattern of the master disk, hence a cause of decrease in the durability of the master disk.
Accordingly, the present inventors have already proposed a master substrate wherein an electroforming layer constituting a metal plate with a surface to which a concavo-convex pattern corresponding to transfer information has been transferred is formed of a multilayer structure of different grain sizes (Japanese Patent Application Laid-Open (JP-A) No. 2006-216204); a master substrate wherein an electroforming layer constituting a metal plate is formed of a three-layer structure in which between first and third layers having the same crystal orientation, a second layer having a different crystal orientation from that of those two layers is sandwiched (Japanese Patent Application Laid-Open (JP-A) No. 2006-221692); a master substrate wherein an electroforming layer constituting a metal plate is formed of two or more layers having different crystal orientations (Japanese Patent Application Laid-Open (JP-A) No. 2006-228316); and so forth.
According to these proposals, the amount of warpage and the amount of distortion can be reduced, and master disks for magnetic transfer which are superior in flatness can therefore be obtained; however, as things stand, provision of a master disk having greater flatness and a smaller variation in the amount of warpage is hoped for to improve transfer adhesion even further.